The invention relates generally to x-ray tubes and, more particularly, to an apparatus for improving heat transfer characteristics in an x-ray tube and a method of making same.
X-ray systems typically include an x-ray tube, a detector, and a bearing assembly to support the x-ray tube and the detector. In operation, an imaging table, on which an object is positioned, is located between the x-ray tube and the detector. The x-ray tube typically emits radiation, such as x-rays, toward the object. The radiation typically passes through the object on the imaging table and impinges on the detector. As radiation passes through the object, internal structures of the object cause spatial variances in the radiation received at the detector. The detector then emits data received, and the system translates the radiation variances into an image, which may be used to evaluate the internal structure of the object. One skilled in the art will recognize that the object may include, but is not limited to, a patient in a medical imaging procedure and an inanimate object as in, for instance, a package in a computed tomography (CT) package scanner.
X-ray tubes include a rotating anode structure for distributing the heat generated at a focal spot. The anode is typically rotated by an induction motor having a cylindrical rotor built into a cantilevered axle that supports a disc-shaped anode target and an iron stator structure with copper windings that surrounds an elongated neck of the x-ray tube. The rotor of the rotating anode assembly is driven by the stator. An x-ray tube cathode provides a focused electron beam that is accelerated across a cathode-to-anode vacuum gap and produces x-rays upon impact with the anode. Because of the high temperatures generated when the electron beam strikes the target, it is typically necessary to rotate the anode assembly at high rotational speed.
Because of the inefficiency of creating x-rays from the focused electron beam, a significant amount of waste heat is generated at the focal spot. One of the major problems in designing and operating an x-ray tube is finding the means to generate the desired amount of x-rays while also removing the waste heat from the focal spot. Typically the peak operating temperature of the focal spot is the limiting factor that dictates the peak power that can be applied at the focal spot. The limiting factor of the focal spot is based on the maximum material temperatures that can be sustained at the focal spot while taking focal track life into consideration. In addition, peak average temperature of the target assembly is also taken into account and can be a limiting factor in operating the x-ray tube, as well. The peak average temperature of the target assembly is dictated by such factors as average overall power, material properties (heat capacity, thermal conductivity, etc. . . . ), and the amount of heat transfer that occurs during operation.
The target assembly is operated in vacuum in order to enable a high voltage potential that can reach 140 keV or greater between the anode and the cathode. Thus, convection heat transfer is not a mode of heat transfer that is available to cool the target within an evacuated region of an x-ray tube. As known in the art bearings in an x-ray tube may include either roller bearings or a spiral groove bearing (SGB). Typically, the contact regions of the bearings are, from a thermal conduction perspective, well removed from the point of heat generation at the focal spot. That is, the conduction path from the focal spot typically includes, first off, passing from the outer radius of the target to the inner radius of the target, then through a thermal barrier, and then along a shaft of the bearing. Thus, regardless of whether the bearing is a roller bearing or an SGB, the conduction path tends to be relatively long and therefore little, if any, significant amount of conduction heat transfer occurs in the target assembly.
Because there is typically little cooling of the target assembly by both conduction and convection, the dominant mode of heat transfer of the target is therefore via radiation heat transfer to its surrounding environment. Thus, design and operation of an x-ray tube presents a daunting challenge, from a thermal perspective, because the dominant mode of heat transfer is by radiation, and because of the dual desire to 1) maximize peak power at the focal spot, and 2) maximize the average power applied to the target assembly.
As known in the art, the average power applied to a target assembly can be greatly increased by providing a lightweight heat storage material, such as graphite, to the back side of the target and approximately opposite the face of the focal track. Graphite has a very high thermal storage capacity and also a high emissivity, enabling a much greater average power to be applied than to a target alone without a heat storage material. However, because the target cap itself typically includes a material such as molybdenum having a relatively low thermal conductivity, and because of the very high localized power that is applied at the focal spot, significant thermal gradients tend to occur within the target cap. Such gradients occur whether the graphite heat storage medium is present or not. Thus, to first order the dual desire to 1) maximize peak power at the focal spot and 2) maximize the average power applied to the target assembly present thermal problems that are independent of one another.
Known solutions to improve the heat transfer performance of x-ray tube targets include the use of low melting temperature phase-change materials or the use of a diamond-metal composite. Such solutions, however, can introduce additional problems that can lead to early life failure, cost excessively in manufacture, and the like. For instance, one known solution includes adding ‘slugs’ of phase-change materials about the circumference of the target and opposite the focal track. Because the slugs heat and cool through a phase-change from solid to liquid, the slugs offer an increased amount of heat storage (relative to the target cap) due to transitioning through the melt temperature of the slug material, taking advantage of the heat of fusion of the slug material. The increased thermal capacity thereby increases the thermal storage of the target, enabling an increased average power to be deposited in the target.
The slugs are placed into a cavity and then sealed in order to prevent leakage during high-temperature operation. However, because of the phase-change that occurs, a volumetric change occurs as well, leading to many high-stress heating and cooling cycles over the life of the x-ray tube, which can lead to significant cycling in the joints used to seal the slugs. Such cycling and stress can lead to failure of the joint(s) that retain the slug(s), causing a loss of the phase-change material and a catastrophic loss of the x-ray tube.
Further, the slugs (whether in liquid or solid phase) typically may not substantially alter the thermal conductivity of the target. As such, substantial thermal gradients tend to occur in the target assembly. Thus, although a phase-change material within the target can increase the overall thermal capacity of the target, it does so at the expense of increased manufacturing cost, added modes of failure, and typically little or no reduction in the thermal gradients that can develop in the x-ray tube target.
Another known solution includes the use of diamond or a diamond-metal composite within the target and positioned under the focal track. Diamond is a material known for its high thermal conductivity. Thus, by introducing diamond or a diamond-based material into a target, thermal gradients may be reduced proximate the focal track providing an improved thermal path from the focal track to, for instance, the side of the target opposite that of the focal track.
However, in order to obtain the increased benefit of the high thermal conductivity material, a high quality bond between the target substrate and the high thermal conductivity material is relied upon. That is, if a poor thermal contact is formed between the material and the target substrate, a tremendous increase in focal spot temperature can result. Typically, diamond or diamond-based materials have a coefficient of thermal expansion (CTE) that is different from the target substrate (typically molybdenum) as well. Diamond-based materials also typically include high elastic moduli as well relative to the base target cap material. Thus, very high stresses can result in the bond joint due to the different CTEs and the high elastic moduli of the attached materials. The very high stresses can lead to bond joint failure, the propensity of which can be exacerbated from the many cycles that occur during the life of an x-ray tube.
In addition, it is commonly known in the art that diamond and diamond-based materials can be very expensive to fabricate and process. Diamond based composites typically consist of discontinuous diamond reinforcement within a matrix of another material. This provides an inefficient thermal path as the diamond particles are isolated and discontinuous within the matrix. Therefore, although diamond and diamond-based materials may provide improved thermal conduction proximate the focal spot and reduced temperature thereof at a given power, their implementation can be quite costly, while adding an additional risk in a new mode of failure.
As such, known solutions for improving thermal performance of x-ray tubes may result in improvement of one performance parameter (i.e., thermal gradient within the target or thermal capacity of the target assembly) while leaving the other parameter generally unaffected. Further, such known solutions typically include an increased material cost as well as an increased cost of manufacturing, while also adding additional failure modes to the x-ray tube.
Therefore, it would be desirable to design an x-ray tube having a target with an improved thermal performance that overcomes the aforementioned drawbacks.